In the prior art, UK patent GB 2071071 describes a process for the preparation of titanium containing silicalite-1 designated as TS-1. The procedure involved in this process is to mix silicon source (preferably silicon tetraethoxide), titanium source (titanium tetraethoxide), organic base (tetrapropyl ammonium hydroxide) and water. The mixture is further heated hydro thermally to obtain microporous material with silicalite-1 structure. Due to difference in the rate of hydrolysis of silicon tetraethoxide and titanium tetraethoxide, formation of sparingly soluble titanium oxide (TiO2) before the formation of titanosilicate, binders the incorporation of titanium in the tetrahedral framework leading to a titanium-depleted material. Also in the prior art, the chemical composition of such titanium silica (TS-1) has been claimed to be xTiO2:(1−x) SiO2 wherein x is in the range of 0.0001 to 0.04. The material has found applications in various oxidation reactions such as the conversion of benzene into phenol, phenol into hydroquinone and catechol, cyclohexanone into cyclohexanone oxime, ethanol into acetaldehyde, isopropanol into acetone, benzyl alcohol into benzaldehyde and benzoic acid etc. To overcome this limitation the silicon and titanium sources are hydrolyzed completely with organic base before mixing to obtain a clear liquid reaction mitre, which results in the titanium rich crystalline material.
U.S. Pat. No. 4,410,501 describes a process for the preparation of titanium silicalite-1 (TS-1). The samples were prepared by hydrothermal crystallization of precursor gel containing. tetraethyl orthosilicate Si (OEt)4 or silica sol (Ludox AS 40) as a silicon source and tetraethyl orthotitanate Ti (OEt)4 as titanium source. The preparation requires first mixing of Ti (OEt)4 in silica source at low temperature (5° C.) followed by the addition of tetrapropyl ammonium hydroxide (TPAOH) solution to perform hydrolysis of the alkoxides. The micrporous crystalline product had the silicalite-1 structure with chemical composition xTiO2 (1×x) SiO2 where x lies between 0.0005 and 0.04. The patent also describes the use of titanium peroxide as titanium source. Tetraethyl titanium is hydrolyzed to obtain white gelatinous suspension, which was cooled at 5° C. and 30% hydrogen peroxide was added to the cooled suspension to obtain orange colored solution of titanium peroxide At this point 25% aqueous tetrapropyl ammonium hydroxide was added at 5° C. After one hour, Ludox colloidal silica, containing 40% of SiO2 was added and the gel was heated in an autoclave at 175° C. for ten days to obtain crystalline TS-1 product. The main drawback in this procedure is the addition of tetrapropyl ammonium hydroxide to titanium peroxide solution, which will decompose the titanium peroxide to titanium oxide/hydroxide, which will hinder the incorporation of titanium into silicalite framework structure yielding low titanium containing TS-1 product with lower catalytic activity. The reactive titanium peroxide species used for the synthesis of TS-1 with high titanium content are destroyed by the addition of TPAOH to titanium peroxide as titanium peroxide is not stable in alkaline medium.
Another process for the preparation (A. Thangiraj, M. J. Eapen, S. Sivasanker and P. Ratnasamy, Zeolites, 12 (1992) 943) involves the complete hydrolysis of silicon tetraethoxide with organic base to obtain clear liquid to which titanium butoxide was added as a titanium source. By this process the titanium containing silicalite-1 was obtained with the chemical composition claimed to be xTiO2 (1−x) SiO2 wherein x is in the range 0.04–0.2. Such titanium rich silicas are found more active in the catalytic conversion of phenol to catechol and hydroquinone as compared to the TS-1 mentioned in the UK patent.
Another process for the preparation of titanium silicalite (A. Thel, Y. Ben Taarit. Applied Catalysis A: General 110, 1994, 137–151) is described using various silicon and titanium alkoxides. Silicon propoxide and silicon butonide whose rate of hydrolysis are very low leading to the formation of poorly crystalline material, containing extra framework species at very low titanium content. No significant influence of the titanium source was observed using silicon ethoxide as silicon source. However h higher titanium incorporation was obtained with silicon methoxide without formation of extra framework species.
In all the above applications, it has been found that catalytic efficiency of TS-1 is directly proportional to the content of titanium in titanium silicalite (TS-1).
The process for the preparation of TS-1 with enhanced catalytic activity and having the chemical formula xTiO2:(1×x) SiO2 wherein x greater than 0.04 and less than or equal to 0.075 described in U.S. Pat. No. 5,885,546 involves the use of acetylacetone forming a soluble complexing agent with titanium source.
In all prior art, efficient incorporation of titanium in silicalite framework i.e, the formation of Si—O—Ti bonds occurs only when it is formed during the reactive gel formation. It is possible only if the rate of hydrolysis of titanium alkoxide matches with that of silicon alkoxide, or silicon alkoxide is first hydrolyzed to obtain polymerized Q1, Q2, Q3 species of Si—OH groups, before titanium alkoxide is added and hydrolyzed in-situ to facilitate Si—O—Ti bonds for this purpose, silicon ethoxide or preferably silicon methoxide should be used. Due to special techniques required for the preparation, higher cost of silicon tetramethoxide/silicon tetraethoxide as well as requirement of higher amount TPAOH as template increases the cost of titanium containing silicalite-1 which is major bottleneck for its commercial utilization.
Copending application No. 3514/DFL/1997 of the applicants discloses a process for preparation of titanium containing silicalite-1 using ethylsilicate-40 and butyl titanate as silicon source and titanium source respectively in isopropyl alcohol and simultaneous hydrolysis using tetrapropyl ammonium hydroxide.